Problem: The scale on a map is 4cm : 6km. If the distance between two cities is 30km, how far apart in cm are the two cities on the map?
The scale means that every 4cm on the map corresponds to an actual distance of 6km. An actual distance of 30km is the same as 5 $\cdot$ 6km. The distance between the two cities on the map is 5 $\cdot$ 4cm, or 20cm.